Reckoned to be wrong, when one considered fully (7,3)
I believe the answer is:
thought out
'considered fully' is the definition.
'thought-out' can be an answer for 'considered' (thesaurus). I am not certain of the 'fully' bit.
'reckoned to be wrong when one' is the wordplay.
I cannot quite understand how this works, but
'reckoned' could be 'thought' ('think' can be a synonym of 'reckon') and 'thought' is found in the answer.
'wrong' could be 'out' (eg a calculation might be 'out') and 'out' is found within the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Another definition for thought out that I've seen is " Planned".)